In the past, many utilities have been using high sulphur coal in their power plants for generating electricity. However, recent clean air legislation has required that sulphur dioxide emissions from power plants be reduced, and this requirement has forced many utilities to switch to natural gas or fuel oil, or to install facilities for stack gas scrubbing. Unfortunately, none of the alternatives has proven to be commercially feasible. Natural gas and fuel oil are more expensive than coal, and in many instances are difficult or impossible to obtain because of shortages. Moreover, stack gas scrubbing processes are still in the development stage.
A better approach to the solution of the problem is the use of coal gas. In the production of coal gas, the sulphur in the coal is converted to hydrogen sulphide which can be readily removed by known adsorption processes. In addition, the coal gas can be produced under pressure permitting operation economy in existing power plants and more efficient design in future power plants, as compared with the coal-burning facilities. Gas produced under pressure from steam and carbon containing materials such as coal or petroleum cake makes an excellent feed stock for chemical processing or hydrogen production.
However, the prior art coal gas production systems require a plant with such a high capital cost that principal, interest, taxes and insurance alone mitigate any cost advantage over the use of coal in its original state. Moreover, when the additional costs of maintenance, operation and coal are added to the foregoing cost, the gas output of the prior art coal plants have a higher cost per unit heat (BTU) than any of the other competitive fuels. The system and apparatus of the present invention, on the other hand, has a dual advantage of low initial cost and lower maintenance. The system and apparatus of the invention is capable of producing a clean, medium BTU gas (300-500 BTU) which is much lower in cost than fuel oil and which is actually competitive with natural gas.
The system disclosed in the copending application includes one or more retorts, the number of retorts used being dictated by the gas requirements of any particular installation. The system of the copending application permits the control of temperature and coal residence time which assists production of methane and higher hydrocarbons. Therefore, a medium BTU coal gas (300-500 BTU) can be produced in the apparatus of the invention without the prior art requirements of an air separation plant for oxygen production and without the prior art requirements for a high pressure system.
The retort used in the apparatus of the copending application, as described therein, is designed with an outer wall in which an annular chamber is provided around the reaction column. The annular chamber may contain steam tubes in which the steam introduced into the reaction column is super-heated by flue gas. By such a construction, the walls of the reaction chamber in the retort are maintained at a higher temperature than the internal temperature of the column, so that any heat transfer through the walls is into the reaction column and not outwardly into the surrounding atmosphere. The retort itself may be modified to accommodate a wide range of different types of coal, and to produce coal gases with varying BTU and chemical compositions to satisfy specific requirements.
Heat will be transferred inwardly from the hot walls of the retort to the reaction column by radiation as long as the walls of the retort are above the reaction temperature in the reaction column. As described above, the apparatus of the copending application uses a retort in which flue gas is used to super-heat steam. This fluid gas passes along the outer surface of the walls of the retort column, so that the walls of the retort are always maintained at a higher temperature than the reaction temperature within the column.
It is often desirable to produce coal gas at high pressure. However, this cannot be achieved directly in the apparatus of the copending application because the hot wall surrounding the reaction column would rupture, if high pressure occurred in the reaction column. The apparatus and system of the present invention is similar to that of the copending application, except that the apparatus and system has been modified to give the retort the capability of producing the product gas at high pressure in a simple, economical, and efficient manner. This is achieved by creating a high pressure in the annular chamber so that the pressure in the reaction column can be raised to a high level without bursting the hot wall around the column.